Many folks do this for a couple of reasons.
On traditional woodburners,you may be wanting to enhance smokering.
At temps down around 170�-180�,your fire may not be as efficient and you may lay on more smoke.
Like Kingfish says,for timing and sleep.
There are also real fine cooks that will finish the same size packer in 8-9 hrs.
I may put it on at 9PM,when I am alert enough to function, and it will be in the plateau around 160�-165�,about 5- 6 AM.
I can then kick my temps up,manage the cook,in the morning.
My practice schedule and comp schedule are pretty close.
We like 15 lb choice packers,in the cryovac,and then trimmed some.
Depending on an assortment of variables,they seem to take about 65-70 mins/lb trimmed weight to finish.
Depending on how they are doing,on my schedule,dictates how high I might kick the heat.
We also like to get them off the heat,at least three hrs ahead of slicing.
Well wrapped and insulated,they may hold 6-7 hrs.
This certainly isn't the only way to time and cook,just one way that works for my schedule.
Hope this helps a little.